Air Barrier

(applied to the inside face of foundation wall above grade)

A layer of building material such as #15 lb. felt paper or Tyvek to restrict the movement of air from the outside to inside. A concrete basement wall is a good air barrier, and does not require an Air Barrier, however a concrete masonry wall is more porous and requires an air barrier. The material must not have a Permeance rating below 3 PERM INS in order to allow the wall assembly to breath.

Air Leakage

This happens due to air pressure difference between indoor and outdoor which forces air through holes in foundation wall above grade.

Dew Point

If air is cooled while maintaining the moisture content constant, the relative humidity will rise until it reaches 100%. This temperature, at which the moisture content in the air will saturate the air, is called the dew point.

In basements warm moisture air will diffuse from the warm interior to the exterior walls. As the air passes through the wall, the temperature will drop and eventually reach the outside temperature. Somewhere through the wall assembly the air will reach the saturation point and if filled with moisture will condensate inside the wall, not a good thing, to prevent this we install a vapour barrier or retardant as there is no perfect barrier to prevent moisture movement outward.

To measure the strength of the vapor barrier or retardant we call it Permeance, the lower the Permeance the better the material. The most common is 6-mil polyethylene with a Permeance of 0.06.

Moisture Barrier

(applied to the inside face of foundation wall below grade)

The purpose is to provide long term protection to the interior wood framing and the batt insulation below grade from moisture. A layer of building material such as #15 lb. felt paper or Tyvek applied to the foundation wall from the basement floor to the exterior grade.


Air travels from warm to cold. In a basement the interior warm air wants to travel by diffusion to the outside. The warm air carries moisture with it, the process is called diffusion. The ability of the wall system to resist the restrict the movement of moisture in through a particular material or wall assembly is called the Permeance.

The lower the Permeance of the material the better it is to prevent diffusion of moisture. The measurement is in Perms. A building material with a Perm rating of 1.0 means 1 grain of water vapor will pass through 1 square feet of the material in 1 hour with the vapor pressure difference of 1 inch of mercury (1 Hg) from the warm side to the cold side.
Note: takes 7,000 grains to create a pound of water.

Material Permeance (Perms)
6 mil polyethylene 0.06
Expanded Polystyrene (ECP) 2.0 to 5.0 per inch (depends on density)
Extruded Polystyrene (EXP) 1.1 per inch
15 lb. tar felt paper 4.0
15 lb. Asphalt Felt paper 1.0
Vapor retarder paint 0.45
Tyvek (Dupont Company) 6 mil 48.0
10" concrete block 2.4

Relative Humidity

Relative Humidity or RH is the measurement by weight per unit volume the of moisture currently in the air expressed as a percentage of totally saturated air for a given temperature.

When the RH exceeds 50% usually in the summer time, the air condensates and creates mold and mildew on personal belongings.

To monitor the RH every basement should have a Hydrometer!
To control the RH every basement should be equipped the dehumidifier preferably an Energy Star appliance.

Thermal Resistance

The R-value or RSI — Value of a material describes its thermal resistance — how much the material inhibits the transfer of heat. The higher the R-value the more effective the level of insulation.

Insulation values for basement walls Range R8 to R19 depending on latitude (postal/Zip codes) and type of fuel or electric heat.

Minimum suggested values for various materials
2 ½" Expanded Polystyrene (EPS) R8.9 (RSI 1.56) Rigid Insulation
2" Extruded Polystyrene (XPS) R10 (RSI 1.76) Rigid Insulation
3 ½" Roxul Insulation R15 (RSI 2.64) Batt insulation

For recommended R or RSI values for basement walls check with local building codes.

In USA go to web site: Insulation fact sheet. And type in Zip Code.

Canada go to web site:_______________

Vapor Barrier (Retardant)

(applied to the warm side of the framed insulated wall)

The job of a Vapor Barrier is to prevent vapor diffusion from the warm to the cold side of a wall assembly. (See also dew point) A vapor barrier is a layer of building material with a perm rating of less than 1 Perm such as 6 mil polyethylene.

Vapor Diffusion

Is the process of moisture passing through breathable building materials such as drywall and batt insulation? Vapor barriers prevent this. Diffusion occurs from warm side to cold side.